1. Field of the Invention
The invention concerns a switching device for supplying power to additional accessories for a telephone device. Power is supplied from the loop current provided to the extension circuit of the telephone device, which defines a load switched in parallel across a pair of signal lines on the extension circuit connected to the telephone device when the device is activated. The distribution of the feed current occurs by means of a current distribution switching means operating in conjunction with the parallel load, at outputs corresponding to the respective additional accessories and assigned priorities for power supply. The supply voltage for the accessories is increased corresponding to increased loop current, and the control device of the telephone unit receives the highest priority, possibly in combination with operation of speech transmitting or receiving means. The loudspeaker and/or hands off device receives a lower priority.
2. Prior Art
A switching device of this general nature is aleady known. A switching device for supplying power to additional accessories for a telephone is described in German Offenlegungsschrift No. 34 04 335, in which the parallel load is defined by a reactance step with constant resistance characteristics. Excess current flows via the parallel load to branched-off loop current including a loop directed to a current stabilizer for a microphone. The parallel load has a current distribution switching means with several power supply outputs, of which one output has the highest priority and the other outputs have declining priorities. The power supply inputs to the additional accessories are connected with these outputs according to their relative importance.
Accordingly, the highest priority can be assigned to the control device, possibly together with speech transmission means, if the latter is not already powered via a current stabilizer. As a rule, the control device requires a relatively low but constant current. The rest of the loop current is made available for the operation of the loudspeaker and/or hands off device.
The control device may have a function requiring supply of power for an extended time, for example including a selection device for transmitting successive digits of a telephone number from a dialing keyboard, in the form of digital switching impulses and/or plural tone signals, and may include a memory for repeated dialing in which the last-dialed telephone number is stored and can be recalled by pressure on a key and again transmitted to the extension connection. In such a system, care must be taken that the telephone number, i.e. the content of the memory, remains stored in the memory at least for a certain time after the handset has been replaced and the supply of current from the switched loop is lost.
An object of the present invention is to provide a switching device for a telephone device of the type having a switched-in parallel load and a current distribution switching means, in which such repeated dialing is still possible for a predetermined time period after replacing the handset.
This object is achieved according to the invention in that the control device has a memory with its own power supply connection, separate from that of the control device in general. This power supply connection is connected to a power supply output of the next lower priority with a capacitor for retaining power to the memory, while the power supply to the loudspeaker and/or hands off device is obtained from the power output with a lower priority.
In this manner, as soon as the handset is lifted the total available excess of loop current can immediately be drawn via the second output of the current distribution switching means to charge the capacitor. One consequence of this arrangement is that during charging of the capacitor, the loudspeaker and/or hands off device receives no current feed. However, this is not disturbing, since the loop currrent begins to flow as soon as the handset has been lifted and the telephone device is switched in. The loudspeaker and/or hands off device can be used after a short charging period of the capacitor.
Should the control device have no separate current supply connection for the memory, a further development of the invention consists in that the supply connection of the control device can be connected with the supply output of the next lower priority and with a memory capacitor via a first Schottky (zener) diode, while the supply to the loudspeaker and/or hands off device is provided by the feed output of lower priority.
Since in this case, the power supply connection to the control device is connected both to the supply output with the highest priority and to the supply output for the next lower priority in the current distribution switching system, the Schottky diode (i.e., zener diode) prevents charging of the memory capacitor from the output with the highest priority.
Further advantages will also be apparent.